1. Field of the Invention
The present invention relates to prostheses adapted to be fixedly attached to bone by means of a bone cement. Specifically, the present invention is directed towards prostheses adapted to maximize the strength and durability of the prostheses/bone cement adherence.
2. Description of the Prior Art
In the field of orthopedic surgery, ZIMALOY, manufactured by Zimmer, U.S.A., Inc., a chromium-cobalt-molybdenum alloy, stainless steel, titanium alloys, and polymerized materials such as ultra high molecular weight polyethylene (hereinafter UHMWPE) have been used successfully to replace the ends of long bones and joints, including the hip joint. However, there exists a severe limitation with respect to such orthopedic surgery, namely, coupling of the protheses to bone. Due to such factors as mechanical stress, fatigue, corrosion, etc., the prostheses/bone cement joints have been prone to failure.
Present methods of utilizing such bone protheses involve the use of a prosthesis having a stem portion which is inserted into the interior of a bone. A bone cement comprising a mixture of polymethylmethacrylate (hereinafter PMMA) polymer and methyl methacrylate monomer (hereinafter MMA) and optionally including a styrene co-polymer of PMMA is likewise inserted into the bone cavity and is utilized to couple the stem of the implant to the bone itself. Experience has demonstrated, however, that serious drawbacks exist with respect to the coupling between the prosthesis stem and the bone cement. Attempted solutions to this problem have been directed primarily toward strengthening the prosthesis/bone cement interface by means of gross mechanical interlock involving, for example, dove tails, small stems, etc. Such devices result in stress concentrations that can exceed the strength of the bone cement as well as cause non-physiological force distribution in the bone.
Adherence at the interface between the implant and PMMA is greatly restricted by current industrial and surgical practices. For instance, the PMMA cement is typically applied in a highly viscous doughy state with the result that the degree of contact between the implant and the cement is inadequate. Moreover, the existence of weak boundary layers such as contaminants and weak metal oxides on the surface of the implant have also caused problems. Weak boundary layers may be due to the composition of the implant or to the process of forming the same. Thus, in the case of a metal implant, the surface of the implant normally includes weak metal oxides as weak boundary layers. In the case of a polymeric implant, the surface of the implant normally includes a weak boundary layer comprising monomer, partially polymerized or low molecular weight polymer and contaminants comprising mold release agents, etc. Finally, the implant may come in contact with air, blood, water, etc. prior to being inserted into the bone thereby becoming contaminated. The existence of weak boundary layers, e.g., surface contaminants, is detrimental to the formation of good implant-bone cement adherence. Thus, the strength of such joints has been dependent upon gross mechanical interlock. Such difficulties in the formation of a satisfactory prosthesis/bone cement connection have also caused the result that mere resurfacing of a deteriorated joint, e.g., a deteriorated hip joint due to arthritis, was not readily accomplished. Thus, in the case of a deteriorated articular surface, e.g., surface of the head or ball in a ball and socket joint, the entire head of the bone is generally removed and a prosthetic head connected to the bone by means of a stem inserted into the interior of the bone, although in some instances, resurfacing implants have been used with bone cement.